This invention relates to insulation for a water cooled member forming a structural portion of a furnace and more particularly to water cooled pipes in reheat furnaces.
In preparation of steel billets, slabs and other metal workpieces for further shaping or forming, it is customary to subject the workpieces to direct heating while passing through a furnace. The billets move through the furnace on skid rails which are supported from below by horizontal and vertical water cooled tubular support members. With this construction, fuel may be burned both above and below the skid rails so that the billets or workpieces may be uniformly heated from all sides.
In the continuous furnace of the type described, the billets or workpieces are generally heated to a temperature on the order of 2000.degree. F. Therefore, the said rails and the support tubes must be protected by sufficient insulating material to minimize the heat lost to the water cooled tubes, and further, to minimize the erosion and corrosion of the tubes resulting from the high temperatures and oxidization environment encountered in the furnace.
In the past, it has been recognized as desirable to insulate these water cooled members by fibrous systems. These systems consisted of applying a lining or covering of fibrous insulating material over the pipes, as shown in U.S. Pat. No. 3,226,101 (Balaz et al) and U.S. Pat. No. 4,095,937 (Colburn et al). However, these systems proved unsatisfactory due to the vulnerability of wet scale attack. Other systems utilized ceramic tiles which were assembled in encircling relationship to the water cooled support members or pipes or a combination of a fibrous inner layer and interlocking ceramic tiles making up the outer layer as shown in U.S. Pat. No. 3,881,864 (Nicol). This system proved more reliable in resisting wet scale attack but were subject to vibration and thermal shock problems which reduced the life of the protective system. In general, however, it had been found that the combination of both fibrous insulation and ceramic tiles covering the fibrous insulation provide the best protection from corrosion caused by furnace gases for the water cooled members. That is, the fibrous insulation thermally insulates the members, and the outer covering of ceramic refractory tiles protect the fibrous insulation from the deleterious effects of the furnace gases. In this combination of insulation and ceramic tiles, the encircling ceramic tiles are subjected to intense vibrational loadings caused by movement of the billets on the skid rail and large thermal stresses as a result of temperature gradients which may cause cracking of the tiles. The internal stresses are more pronounced in the vicinity of the engagement means employed to interlock adjacent tiles or members. In general, the aforementioned engagement means consists of a metallic stud welded to the tubular support member and for interlocking lug members formed in the ceramic tiles, such as shown in U.S. Pat. No. 2,436,452 (Schmidt). Furthermore, the complex arrangement generally employed in holding the ceramic tile members together about the tubular support member renders replacement of a single ceramic section quite difficult.
Thereafter, a covering was developed which insulated the tubular members, which protected the insulation from the furnace gases, and minimized the magnitude of internal stress imposed thereon. This covering consists of thermal insulating layer of ceramic fiber wrapped around the fluid conveying tubular member, an inner ceramic refractory layer of split tiles and an outer layer of ceramic refractory tile forming a unitary collar, as shown in U.S. Pat. No. 4,015,636 (Van Fossen). However, the high density of this covering system renders it vulnerable to intense vibrational loading.
Therefore, it is the object of this invention to provide a covering for water cooled members in a furnace which has good thermal insulating properties, which is resistant to scale track and internal stresses and which has a low density in order to avoid vibrational loading stresses caused by movement of the billets on the skid rails.
In accordance with the present invention the water cooled furnace support members are provided with a refractory covering comprising a thermal insulating ceramic blanket layer disposed about and contacting the member, a ceramic or fiberglass cloth wrapped about the blanket and a veneering mortar layer applied thereon, a layer of rings or strips of a ceramic fiber material is disposed about and contacting the veneering mortar, and a hot face veneering coating layer is applied to the rings of ceramic fiber material.
Specifically, the low density refractory covering for the water cooled furnace members includes a thermal insulation material such as a ceramic fiber blanket wrapped around the member to be insulated. Ceramic fiber cloth woven in an open weave ("fish net"), is wrapped about the thermal insulation blanket in order to hold it onto the insulated member. The weave must be such that it allows penetration of the cloth by an alumina-silica high temperature fiber veneering mortar. The mortar is generally sprayed or troweled onto the cloth, and thereafter penetrates into the ceramic fiber blanket. Rings of ceramic fiber material are placed atop the unset mortar. These rings are wrapped in a compressed relationship with respect to each other so as to take into consideration any shrinkage which may occur. Alternatively, strips of ceramic fiber blanket can be spirally wrapped around the veneering mortar. The rings or strips of ceramic fiber are applied before the veneering mortar has set, therefore the rings or strips are held in place by adhesion to the mortar while a final hot face veneering coating is applied. The hot face coating has a high alumina content, the composition being designed for use with the particular ceramic fiber material chosen to be used as rings or strips. It is important that this coating is compatible with the ceramic fiber because shrinkage of the fibers during heating would cause the coating to flake off.